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Discussion Starter #1
I have a Trane gas furnace with a air-conditioning combination unit.
The Trane gas furnace model is TDD140R960C5 around 8 years old. The furnace Controller is White Roger Emerson, Model 50A51-495. Yesterday, the furnace stopped working. I tested the furnace. I found after the thermostat turned on the furnace, the controller clicked a sequences and the glow igniter device turned on three times. The small blower at the exhaust turned on. However, the gas valve was not turned on. At the end of the sequence the blower turn on, but was blowing un-heated air. I noticed the controller LED did not blink any special sequence, just normal slow flashing. I test my gas water heater, sitting next to the furnace and the heater worked fine. Today, the furnace is working again.

I suggest suspect the valve is intermittent. Can you please tell me, according to my picture:
What kind of gas valve I have? I assume this is the unit labeled “R”. How to test if this valve is faulty, online?
What is the function of this small fan at the exhaust?
What is this two round metal cans, labeled “P02 Low” and P01 HI”?
 

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When you have furnace failures without a led diagnostic flag it usually means

The thermostat is erratic. Remove stat lines in furnace to R & W. Pigtail R to W to bypass thermostat circuit.

Your board is faulty and perhaps poor grounds can be found and improved.

Your board is faulty and needs to be changed.
 

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They open and close on 24 volts. You'd need to see if the board is sending 24 volts to the valve during a call for heat. Check the plug for voltage by removing it from the valve and sticking your two leads of a meter into them. Unless you have spade terminals but it doesn't appear so.

You should hear the valve give a distinct click when it receives voltage, as it opens if it's plugged in and receiving said voltage.

Does the steel gas pipe leading into the furnace have a drip leg on it, an upside T? Many times debris in the gas line can clog up the valve.

You need a manometer to test gas pressure to and through the valve.
 

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no disrespect...but based on your discription of components on your furnace and the questions posed to us....it would be best to have it serviced by a pro thats imo only...if you feel yo can do this follow how's advice on jumping r-w on control board to by pass thermostat to see if it is the problem..ben sr
 

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Discussion Starter #5
Pardon, what is R and W?

No, I did not hear any click at the gas valve (which is labelled "R", correct?)
There is a ON-Off switch on top of the gas valve. What is the purpose of this ON-Off switch?

What is the function of two round metal cans, labeled “P02 Low” and P01 HI”?

The local HVAC guy said they charge min $150 to come out and look. Now my furnace is working again this morning, they can only guess what kind of problem my furnace has. He suggest better wait until the furnace stop working, then, call him to come out.
 

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Perhaps if this happens again. Turn off power to the furnace and remove the stat wires attached to the R & W at the thermostat. Twist those two wires together and turn the power back on.
If the furnace works when the stat is bypassed in this way then the thermostat is the fault.
 

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Discussion Starter #8
thermostat is working fine, check the AC side and it always works OK. At this time, the thermostat do not have any problem at all.

attached is the schematic for the furnace
 

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Discussion Starter #9
well, the furnace stopped again. The White Rogers Model 50A51-495 controller LED is flashing 2 times = system lockout / no flame / check line polarity. What does check line polarity means?

I turn the power off. Then, knock the Gas valve couples times with a screw driver. Turn the system power on. The furnace is now running again.

Is there anyway to test and oil this gas valve?
Looking at the valve picture, what is the exact manufacturer and model number?
 
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